There is considerable interest in materials with low dielectric constants for use in integrated circuit manufacturing. Integrated circuits consist primarily of transistors and other devices interconnected by wires. The wires are separated from other wires and from the integrated circuit substrate by dielectric films which must be deposited onto the integrated circuit during its manufacturing process. The common dielectric material used in integrated circuits was for decades silicon dioxide, whose dielectric constant k lies between 3.9 and 4.2. Generally speaking the capacitance of wires to ground and to other wires in an integrated circuit will be proportional to the dielectric constant of the dielectric material which separates them. The time for a signal to propagate over a wire in an integrated circuit is related to the product RC, R being the resistance of the wire and C its capacitance to ground. Thus, a reduction of the dielectric constant, leading to a reduction in C, would speed signal propagation and so would tend to make integrated circuits faster. A reduction in dielectric constant would also reduce the power required for signal propagation, which is also approximately proportional to C. Because of this, it is desirable to manufacture integrated circuits which use a dielectric with a significantly lower dielectric constant than silicon dioxide.
To achieve a modest improvement in dielectric constant, silicon dioxide has been replaced in some cases by fluorinated silicon dioxide, resulting in k=3.6-3.8. Lower k alternatives are planned for the 90 nm generation of integrated circuit processes. The dielectric in many such processes will be a carbon-doped oxide (CDO) deposited by CVD deposition of silicon-containing precursors. Carbon-doped oxides will deliver dielectric constants ranging from 2.9-3.1 depending on the precursor and process.
Among the techniques which have been studied for obtaining materials with a lower dielectric constant for use in integrated circuit fabrication is the formation of porous materials. The pores are filled with air and thus reduce the dielectric constant of the overall material, given that the dielectric constant of air is about 1.01.
Generally speaking, porosity in a thin film can be introduced by a variety of means, for example by introducing free volume, by packing shape persistent objects, by using porous matrix particles, or by the introduction of sacrificial pore generators (porogens).
Porogen materials which have been studied include isolated small molecules (cyclodextrins, calixarenes), self-assembled supramolecular structures (e.g., surfactants forming micelles), or polymeric materials. See in this regard W. Volksen et al., “Porous Organosilicates for On-Chip Applications: Dielectric Generational Extendibity by the Introduction of Porosity,” in Low Dielectric Constant Materials for IC Applications, P. S. Ho, J. Leu, W. W. Lee eds., chapter 6 (Springer-Verlag 2002); J. L. Hedrick et al., Adv. Mater. 1998, 10(13), 993; J. L. Hedrick et al., Chem. Eur. J. 2002, 8(15), 3309; J.-H. Yim et al., Adv. Funct. Mater. 2003, 13, 382 (discussing cyclodextrins); P. J. Bruinsma et al., Proc. Mater. Res. Soc. 1997, 443, 105; C. J. Brinker et al., Adv. Mater. 1999, 11(7), 579; D. Zhao et al., Science 1998, 279, 548; D. Zhao et al., Adv. Mater. 1998, 10(16), 1380.
Porogens would generally be used in the following manner. Precursors to a suitable dielectric matrix would be mixed in particular proportions with the porogen. The matrix precursors would then be processed to form the matrix, commonly undergoing some sort of vitrification. Once the matrix is formed, the porogens would be eliminated, typically by heating (burnout), leaving voids. Matrices for the manufacture of dielectrics can be, for example, inorganic thermosets, silsesquioxanes, organic silicas, or organic thermosetting resins, such as SiLK (Dow Chemical Company).
Polymeric materials can give rise to pores in a matrix in at least two ways. One way involves nucleation and growth (N&G) with kinetically arrested domain formation. For this route, the polymer must be initially miscible in the matrix polymer but becomes immiscible during the vitrification of the matrix resin, resulting in the formation of small domains of isolated pore-generating polymer. With appropriate processing, the growth of these nanodomains is arrested by the vitrifying matrix and pore sizes remain small when the polymer is removed via burnout. Porogens in an N&G approach can be simple linear homo and block copolymers if the incorporated functionality interacts strongly with that of the resin precursor. Where there is a weaker interaction, the porogens are preferably multi-armed stars to improve resin compatibility. The ultimate size of the pores depends on porogen structure, molecular weight and loading level, the nature of the matrix resin prepolymer, and the processing conditions. In general, the number of variables in play makes N&G procedures difficult to control.
A second approach to the formation of pores utilizes templating porogens to direct the vitrification of the matrix around the porogen. Templating porogens often lead to more regular structures and are, in general, simpler to control.
A type of templating process which has been studied involves dynamic self-assembly of the porogen into regular structures within the matrix which template the vitrification of organosilicate monomers. An example of this type of process would be the formation of mesoporous silica formed using monomeric or polymeric surfactants.
Alternately, vitrification may be templated with polymers which are either true nanoparticles because they are crosslinked, or which show particle-like behavior. In these cases, the particles are not strictly speaking miscible with the matrix resin, but they are compatibilized with that resin in order to prevent them from experiencing macroscopic aggregation. Crosslinked particles produced by microemulsion polymerization constitute examples of this approach which have been experimented on in both organosilicates and organic polymers. See in this regard M. Antonietti et al., Angew. Chem. Int. Ed. 1998, 27, 1743; D. Mecerreyes et al., Adv. Mater. 2001, 13(3), 204; U.S. Pat. No. 6,391,932 to R. H. Gore et al.; U.S. Pat. No. 6,420,441 to C. S. Allen; U.S. Pat. No. 6,271,273 to Y. You et al.; WO 00/31183 to K. J. Bruza et al.; U.S. Pat. No. 6,653,358 to K. J. Bruza et al. When employing crosslinked particles as templates, judicious control of the functionality by controlling the sequence of monomer addition and reaction conditions is employed in order to produce adequately small particles with functionality which is compatible with the matrix material. Highly crosslinked particles often have a significant char yield which effects the intrinsic efficiency of pore generation.
Materials which are not highly crosslinked may exhibit particle-like behavior and be suitable as porogens. In particular, soft colloids can also display particle-like behavior in polymeric matrices without the need for extensive crosslinking. See in this regard E. F. Connor et al., Angew. Chem. Int. Ed. 2003, 42(32), 3785; R. D. Miller et al. in Polymers for Microelectronics and Nanoelectronics, Q. Lin, R. A. Pearson, J. C. Hedrick eds. (ACS Symposium Ser. No. 874, American Chemical Soc. 2004). It is a feature of this approach that a single porogen particle results in a single pore, so there is no need to achieve control of aggregation or precipitation dynamics of porogen particles. In this approach it is important that the porogens remain dispersed during matrix formation and do not aggregate.
In this approach, particles used as porogens are preferably unimolecular polymeric amphiphiles with a core which is highly incompatible with the matrix resin through the whole vitrification process and an outer corona which is compatible with that resin. With an amphiphile of this type, even though the core collapses into a dense ball, the amphiphilic particles may tend to remain dispersed because of the interaction of the corona with the matrix resin. In this situation, the ultimate size of the pore could either be that of the collapsed core alone, if the corona stays miscible throughout vitrification, or that of the entire particle assuming that the corona also collapses and becomes immiscible as the matrix vitrifies. U.S. Pat. No. 6,399,666 describes the use of certain unimolecular polymeric amphiphiles as porogens in organosilicate matrix materials.
The amphiphilic particles of U.S. Pat. No. 6,399,666 were produced using tandem polymerization procedures. Other references relevant to successive tandem polymerization include R. D. Miller et al. in Polymers for Microelectronics and Nanoelectronics, Q. Lin, R. A. Pearson, J. C. Hedrick eds. (ACS Symposium Ser. No. 874, American Chemical Soc. 2004); J. L. Hedrick et al., Macromolecules 1998, 31, 8691; S. Angot et al., Macromolecules 1998, 31, 7218; and J. Ueda et al., Macromolecules 1998, 31, 6762.
There are some drawbacks to the use of tandem polymerization techniques to produce amphiphilic porogens. First, to make tandem polymerization highly efficient one should achieve good control of molecular weights and polydispersities for two consecutive polymerizations. This is potentially difficult to achieve. Such control is particularly needed where the intent is to use the resulting particles in a templating process where one particle produces one pore and the porosity size distribution mirrors the molecular weight distribution of the particles. Second, the latent initiator sites for the second polymerization must be carried unchanged through the first polymerization and must remain available for transformation into the active initiator for the second polymerization. Typically, some of these sites are lost during the first polymerization and are therefore unavailable for the second. Third, the scale up of any process involving two consecutive polymerizations is difficult because it requires two yield-reducing polymer purification steps to separate the desired product from homopolymer and other contaminants. When carried out on a large scale, this may require multiple solvent precipitations from large volumes of solvent. Fourth, successive tandem polymerization is most appropriate for applications that require particle sizes in the 25-75 nm range. Difficulty is experienced in achieving lower particle sizes, perhaps on account of the difficulty of controlling a polymerization to generate many arms, each with very low degrees of polymerization, particularly when using core-out procedures. In addition, it is common to use atom transfer radical polymerization procedures (ATRP) to produce the corona using polar monomers such as substituted acrylates and methacrylates. (For background on the use of ATRP see T. E. Patton et al., Adv. Mater. 1998, 10(12), 901.) Although polar coronas can be produced with ATRP, this polarity is introduced by the monomer side chains (PEG, hydroxyethyl, etc.) while the backbone remains relatively hydrophobic, diluting the effect of the polar side chain. The result is that the degree of polymerization of each arm in the corona must be increased make the strongly hydrophobic core compatible in polar matrix media.
For these reasons, there is a need for alternative synthetic routes for producing unimolecular porogens which overcome the drawbacks of tandem polymerization and other available techniques.